Polyethylene as a homopolymer or an ethylene alpha olefin copolymer can be employed in a number of commercial applications in which good visual or optical properties are important. For example, polyethylene may be employed in the production of various products such as bottles or other containers and the like which can be produced by blow molding or extrusion molding operations. In such applications, it is desirable to arrive at a product having good optical characteristics in which a desired color is maintained without extensive yellowing of the bottle or other container with time. The resistance of a polymer product to yellowing with time can be measured by the Yellowness Index (YI) as determined in accordance with American Society for Testing Material Standard ASTM-D 1925. Other optical properties which can be important include haze as determined in accordance with ASTMD1003 and gloss as determined in accordance with ASTMD2457.
Significant physical characteristics of polyethylene polymers include the density, the molecular weight distribution, MWD (a ratio of the weight average molecular weight, Mw, to the number average molecular weight, Mn), melt indices MI2, MI5, HLMI and shear response as determined by the ratio of melt indices as determined in accordance with standard ASTM D1238. Thus, the shear response, SR2, is characterized as a ratio of the high load melt index (HLMI) to the melt index MI2 and the shear response, SR5, is the ratio of the high load melt index to the melt index MI5. The various melt indices are conventionally reported in terms of melt flows in grams/10 minutes (g/10 min.) or the equivalent measure as expressed in terms of decigrams/minute (dg/min.). In the polymerization of ethylene to produce ethylene homopolymers and copolymers, a feedstream comprising ethylene, and optionally a higher molecular weight olefin such as hexene, is supplied to a polymerization reactor along with a polymerization catalyst. The polymerization catalyst may take the form of a Ziegler-Natta catalyst, a metallocene-based catalyst, or a chromium-based catalyst, sometimes referred to as a “Phillips-type” catalyst. Such catalysts typically are supported catalysts which are supplied to the polymerization reactor in particulate form along with a co-catalyst which may be incorporated with the supported catalyst or supplied separately to the polymerization reactor. Co-catalysts, which are employed to activate the primary catalyst, include alkylalumoxanes such as methylalumoxane or tri-alkylaluminums such as triethyl-aluminum in the case of Ziegler-Natta and metallocene catalysts and triethyleborane in the case of the chromium-based catalysts.
The polymer fluff withdrawn from the polymerization reactor is typically separated from the diluent in which the polymerization reaction proceeds, and then melted and extruded to produce particles of the polymer product, typically in the nature of pellets having dimensions of about ⅛″-¼″ which then are ultimately used to produce the polyethylene containers or other commercial products. During the extrusion process, stabilizing agents may be incorporated into the polymer. Such stabilizing agents typically include phenolic antioxidants, such as sterically-hindered phenols and phosphite antioxidants. Other polymer characteristics which are significant in terms of suitability of the polymer for the end product include resistance to mechanical failure as measured by notched constant ligament stress (NCLS) and environmental stress crack resistance (ESCR) as determined in accordance with American Society Testing Standard ASTM D1693.